Copper-containing amine oxidases (AOs), enzymes ubiquitous in organisms from bacteria to mammals, are involved in the metabolism of amines. These enzymes are among a rapidly growing class of enzymes that contain a post- translationally modified active site cofactor. In AOs, the cofactor, topa quinone, is formed from a tyrosine residue in a highly-specific, self- processing reaction that requires both copper and dioxygen via an unknown mechanism. In this study, the mechanism of topa formation will be probed using a variety of techniques, such as stopped-flow absorption spectroscopy, rapid freeze-quench isolation of intermediates, kinetic analysis, isotope labeling studies, site directed mutagenesis, and spectroscopic analysis by resonance Raman, circular dichroism, magnetic circular dichroism (MCD), fluorescence, and electron paramagnetic resonance (EPR). The copper binding site in the unprocessed enzyme (before topa is generated) will also be examined using the spectroscopic techniques that have already been used to examine the copper binding site of the active (topa-containing) enzyme (EPR, MCD, electron spin echo envelope modulation, electron nuclear double resonance, extended X-ray absorption fine structure, UV-vis). X-ray crystal structures of unprocessed of AOs will also be obtained. These studies will be done on both a prokaryotic and eukaryotic AOs. Comparison of the copper binding sites before and after topa has formed, comparison of the mechanisms of topa formation in these enzymes, and examination of the requirements for topa formation will lead to a deeper understanding of protein folding and mechanisms of post-translational amino acid modification in enzymes.